Conventional, full color three-dimensional video techniques usually involve the use of glasses. These systems work by alternating left and right eye views on a display such as a cathode ray tube (CRT). The images are separated by looking at them through two sets of filters so that, with interlaced video, the odd line scan corresponds with one eye's view and the even line scan corresponds with the other eye's view. One way of accomplishing the filtering is to put a fixed polarizer on the CRT and to look at the screen with active polarizer glasses. The polarizations of lenses of the glasses alternate out of phase with each other in synchronization with the vertical synch signal of the CRT. Thus, only one eye can see the CRT at one time. The other lens is cross-polarized to the screen polarizer so that nothing is seen with that eye. The same result can also be obtained by placing an alternating polarizer in front of the screen and fixed, perpendicular polarizers in the glasses. Glasses are not only unpleasant but restrict the viewer to a single stereo scene comprised of one view for each eye. A further drawback of such systems is that they are based on assumptions with respect to the viewer's visual system for their effect. A viewer with an unusual interocular distance, for example, would experience depth distortion with a conventional stereo pair.
Auto-stereoscopic systems avoid the requirement for glasses and thus some of the attendant drawbacks of glasses-dependent systems. An example is the holographic method. Holography reproduces both the phase and amplitude of the wavefront coming through a window defined by the hologram in order to exactly duplicate all of the light scattered from a scene and all of the depth information contained within that light. To paraphrase Emmett Leith, one of the fathers of holography, it is as if the light had "fallen asleep" on the hologram during its construction and "wakes up" when it is illuminated for display. Unfortunately, holograms of three dimensional objects must be made with strictly coherent illumination. Further, if the objects move more than a fraction of a wavelength during exposure, the fringes of the hologram are blurred out. These severe drawbacks have prevented holography's development as a mass medium. Also, the incredibly high spatial resolution of holograms on the order of 1,000 lines per millimeter has prevented their real-time electronic transmission.
Other available auto-stereoscopic media include integral photography and spatially multiplexed parallax barriers. While both of these utilize incoherent light, they have relatively strict limitations resulting from the optics involved. Conventional spatially multiplexed parallax barriers use wires or other means so that views are imaged at appropriate angles. Because all views must be presented simultaneously, the number of views is fairly limited. Also, the user must hold his head in a fixed position to see a correct orthoscopic view. Integral photography uses lenses to overcome this difficulty to some degree but introduces blurring, distortion and other problems. Furthermore, it has proven difficult to adapt such a system to electronic imaging because of alignment and imaging problems.